Avoid usage of PATH_MAX.

Taken from https://salsa.debian.org/freedesktop-team/fontconfig/-/blob/master/debian/patches/path_max.patch

Index: fontconfig-2.13.1/src/fccfg.c
===================================================================
--- fontconfig-2.13.1.orig/src/fccfg.c
+++ fontconfig-2.13.1/src/fccfg.c
@@ -2231,7 +2231,7 @@ FcConfigRealFilename (FcConfig		*config,
 
     if (n)
     {
-	FcChar8 buf[PATH_MAX];
+	FcChar8 buf[FC_PATH_MAX];
 	ssize_t len;
 
 	if (sysroot)
